This invention describes porous ceramic fiber insulating materials and methods for their production. More specifically, the invention relates to ceramic fiber insulating materials for use as a high temperature thermal protection system on commercial, military and space vehicles.
Reusable launch vehicles (RLVs), such as the space shuttle, repeatedly travel into or beyond the Earth""s upper atmosphere and then return to the Earth""s surface. During flight, the RLVs experience extreme temperatures, ranging from xe2x88x92250xc2x0 F. while in orbit to over 3000xc2x0 F. upon reentry to the atmosphere. Because of the extreme temperatures, the vehicle and its compartments must be protected by a thermal protection system. The thermal protection system is an outer covering of insulation, the purpose of which is to prevent the body of the vehicle from reaching a certain maximum temperature. For the space shuttle, the maximum temperature is about 450xc2x0 F., the temperature at which the aluminum structure of the shuttle begins to weaken.
Thermal protection systems for RLVs are constructed from a large number, usually several thousand, of insulative rigid tiles and blankets. The tiles, which are used mostly on the lower surface due to their smoother surface, function to insulate the vehicle from the environment and to radiate and reflect heat from the vehicle. In addition to protecting the vehicle from environmental heat sources, the insulative tiles also provide protection from localized heating from sources such as the vehicle""s main engine, rocket boosters and directional thrusters.
RLVs such as the space shuttle typically utilize a variety of tiles to cover the lower surface of the vehicle. Different areas of the vehicle encounter different heat profiles and different physical stresses during flight. Therefore, a variety of tiles having different shapes, sizes, compositions, densities, and coatings are placed at different positions of the vehicle depending on whether such positions are leeward or windward, upper or lower surfaces, etc. The most predominate tiles used today on lower surface are 9 lb/ft3 Lockheed Insulation (LI-900). Alumina Enhanced Thermal Barrier 8 lb/ft3 (AETB-8) tiles are used on the base heat shield due to their relatively higher thermal conductivity and better durability.
The Lockheed Insulation materials are comprised of high purity amorphous silica fiber. To produce the Lockheed Insulation, silica fibers having a diameter of 1 to 3 xcexcm are mixed with de-ionized water in a v-blender and thereby form a slurry. The slurry is mixed with ammonia and stabilized colloidal silica solution after which, it is placed in a casting tower where it is dewatered and slightly pressed to remove a portion of the water. The partially dried slurry is heated to a temperature of 250xc2x0 F. to remove the remaining residual water. The dried silica composition is then fired to a temperature of up to 2300xc2x0 F., which causes the colloidal silica to sinter the fibers to one another. The resulting insulative material is a low-density mass of randomly arranged fused silica fibers. By selectively pressing the silica fiber slurry and subjecting to different firing temperatures, various densities of the rigidized silica fibers may be produced. The Lockheed Insulation tiles are marketed under the trade names LI-900(trademark), LI-1500(trademark) and LI-2200(trademark), having densities of 9 lb/ft3, 15 lb/ft3 and 22 lb/ft3, respectively.
The Alumina Enhanced Thermal Barrier (AETB), invented by NASA Ames Research Center, consists of about 68 percent silica fiber, about 12 percent Nextel fiber (a combination of alumina, silica, and borate), about 20 percent alumina fiber, and about 2 percent silicon carbide powder. The fiber diameter ranges from 1 to 3 xcexcm for silica and alumina fibers, and from 5 to 10 micron for Nextel fibers. The processing is very similar to that used for the Lockheed Insulation. Colloidal silica is not added to the AETB material before firing. Instead, high temperatures experienced during firing cause the borate contained in the Nextel fiber to form boron oxide, which fuses to the fibers and sinters the ceramic fibers to one another. The AETB material is commonly marketed in the forms of AETB-8(trademark), AETB-12(trademark), and AETB-20(trademark) tiles, having densities of 8 lb/ft3 , 12 lb/ft3, and 20 lb/ft3 respectively.
Because of its extraordinary low thermal conductivity, LI-900(trademark) insulation tiles are used on a majority of the lower surface of the space shuttle. Reaction Cured Glass (RCG) is applied on the outer surface of LI-900(trademark) tiles to emit high heat encountered during reentry into the Earth""s atmosphere. LI-900(trademark) insulation, however, suffers from two main disadvantages. First, it suffers from severe shrinkage when exposed to temperatures above 2500xc2x0 F. for long periods of time. Shrinkage along the outer mold line of thermal protection materials leads to widening gaps as well as forming surface recession and thus increases localized heating at the inner substructure mold line. Second, LI-900(trademark) and other pure-silica tile insulations are not compatible with the tough coating, TUFI (Toughened Unipiece Fibrous Insulation), which is needed for improved surface durability. Application of TUFI coating results in slumping of the pure silica insulation. Because of incompatibility with the tough coating, LI-900(trademark) materials are easily susceptible to damage during flight or servicing of the RLV.
Unlike LI-900(trademark) insulation, the AETB material is compatible with the TUFI coating. As a result, the AETB is a much more durable tile system, which requires less frequent replacement. AETB, however, is more thermally conductive than the LI-900 tile materials. As a result of the increased thermal conductivity, the AETB material can only be used in benign areas such as base heat shield. Therefore, AETB may not be used on the lower surface of the RLVs.
What is needed is a ceramic fiber insulative material having the same or lower thermal conductivity found in LI-900(trademark) insulation while exhibiting the durability, dimensional stability, and strength of AETB.
The present invention is an insulating material for use in extreme temperatures up to 2400xc2x0 F. for multiple use, and 2700xc2x0 F. for single use. The tile materials can be used in a variety of applications. The insulating material is a unique combination of ceramic fibers, which are sintered together to form a low density, highly porous material with very low thermal conductivity. The new tile insulation exhibits a high tensile strength, and outstanding dimensional stability and durability to withstand damage typically suffered during flight and servicing of the RLV.
The basis of the invention is the combination of silica (SiO2) and alumina (Al2O3) fibers, and boron-containing powder that are used as a sintering agent. The insulative material is composed of about 60 wt % to about 80 wt % silica fibers, about 20 wt % to about 40 wt % alumina fibers, and about 0.1 wt % to about 1.0 wt % boron-containing powder.
During processing, the boron-containing powder provides boron-containing by-products which fuse and sinter the silica and alumina fibers when heated to elevated temperatures. Thus, no supplemental binder is required during production of the insulative material. It has been found that use of the boron-containing powder allows the use of lower amounts (relative to Nextel fibers used in AETB production) to form sufficient sintering between the fibers. This small amount of boron-containing powder is replacing a relatively large amount of Nextel fibers (12 to 15 wt %), which is one of the high cost components and is found to provide adverse effects on the thermal conductivity due to its larger diameter.
The new tile material is produced by dispersing the ceramic fibers in an aqueous solution forming a slurry. The slurry is blended using the shear mixer, which disperses the fibers evenly throughout and chops them to a certain length. By using a shear mixer, the fibers tend to be oriented lengthwise in the direction of the radial flow of the slurry during mixing. In the finished tile, the fibers are substantially oriented in the direction perpendicular to the press direction of the slurry, making this material anisotropic. This arrangement of fibers results in much lower thermal conductivity along the press direction (through-the-thickness) relative to the direction perpendicular to the press direction (in-plane).
After mixing and chopping, the slurry is optionally classified through a separation means in order to remove undesirable solids, known as inclusions or shot, from the fiber slurry suspension. The insulative characteristic of the material stems from having small diameter ceramic fibers surrounded by large volumes of air. High-density ceramic shot or clumps are detrimental to the effectiveness of the insulation properties, and are therefore removed before the material is pressed.
After filtration of the shot and/or clumps, the slurry is pumped into the mold, otherwise known as the casting box, from which the fibers are drained and pressed. Water removal is accomplished via gravity drain through the porous bottom of the casting box. Acceleration of the draining step is done by the application of a vacuum at the bottom of the casting box. The slurry is pressed to produce a wet billet of ceramic fiber. The slurry is preferably pressed in the vertical direction, by moving a top surface downwards and pressing upon the fibers. The vertical press direction is also called xe2x80x9cthrough-the-thicknessxe2x80x9d direction. The geometry of the top surface, otherwise known as the press plate, is preferably similar to that of the billet to reduce, if not eliminate, fiber layer separation caused by surface friction with the inner walls of the casting box.
After pressing, the wet billet is dried and fired. The drying step removes residual moisture from the billet. The firing step fuses the fibers to one another to produce a rigid body and to provide structural integrity. Drying occurs at approximately 200 to 500xc2x0 F. for at least 24 hours. Firing occurs at a temperature between about 2300xc2x0 F. and about 2600xc2x0 F. for about 1 to about 5 hours.
The fused insulative material is finally machined into the shape of a tile, normally in the 6-inch by 6-inch planform and with thickness ranging from 1 to 3 inches. The tile is machined so that the top surface and the bottom surface of the tile are roughly parallel to the direction of the fiber alignment within the tile material. This arrangement provides an increase in tensile strength in the in-plane direction, which prevents the shrinkage and slumping that is problematic in the older generation tiles. For example, tensile strength of a new tile having a bulk density of 8 lbs/ft3 is approximately 110-140 lbs/ in2 in the in-plane direction and approximately 35-55 lbs/in2 in the through-the-thickness direction. The direction is termed as xe2x80x9cin-planexe2x80x9d when it is perpendicular to the fiber press direction, while xe2x80x9cthrough-the-thicknessxe2x80x9d direction is termed when it is parallel to the fiber press direction. The strength of the tile is sufficient to support a reaction-cured glass (RCG) and TUFI coating applied on the outer surface of the tile without problems associated with slumping.
The insulative material exhibits very low thermal conductivity, particularly in the through-the-thickness direction. One of the primary reasons for low thermal conductivity is due to the fibers which are preferentially aligned in the in-plane direction, and thus substantially reducing the thermal conductivity in the through-the-thickness direction. The other reason is due to the finer diameter of SiO2 and Al2O3 fibers promoting radiation scattering more effectively compared to the fibers having relatively larger diameter such as Nextel fibers.